This invention relates to a method to restore the metal content of a supported noble metal hydrogenation catalyst comprising adding the appropriate amount of a noble metal salt of a weak acid to a fluid feed passing across the catalyst. For example, palladium content of the catalyst used for hydrogenation of phenol can be restored by adding the appropriate amount of a weak acid salt of palladium, such as palladium phenate, in the phenol before it passes over the catalyst.
Noble metal catalysts, such as palladium, are well known in the prior art for various reactions. See U.S. Pat. Nos. 2,829,166; 2,857,337; 3,076,810; 3,305,586; 3,457,187; 3,356,729; 4,203,923 and 4,152,291, all of which are hereby incorporated by reference. Also, British Pat. No. 967,919 discloses hydrogenating benzoic acid to hexahydrobenzoic acid. Other noble metal catalyzed processes are disclosed in U.S. Pat. Nos. 4,242,235 (silver); 4,171,288 (platinum group); 4,147,660 (platinum group); 4,145,314 (Group VIII metal group) and 4,122,040 (platinum group), also incorporated by reference.
Previously, as explained at the bottom of column 1 to column 2 of U.S. Pat. No. 3,305,586, it was necessary to rejuvenate catalyst by adding fresh, virgin catalyst to the reaction mass, or to counteract poisons with promoters as disclosed in U.S. Pat. No. 3,076,810. Promoters are also included in this invention and would supplement it. For example, a hydrogenation of phenol to cyclohexanone involves a palladium catalyzed reaction between phenol and hydrogen. Production capacity and also the amount of undesirable by-product formation depend primarily on the quality of the palladium catalyst which is present in the continuous process loop. Feedstock contaminants capable of irreversible adsorption on the catalyst deactivate the catalyst. Loss of palladium from the surface of the catalyst renders the catalyst less effective. Presently, the quality of the catalyst is maintained through a catalyst purge and make-up procedure. This method has several disadvantages. Other rejuvenation procedures also are impractical.
The catalyst, 5% (as charged) palladium on carbon, in the phenol hydrogenation process loses activity and selectivity. This deterioration is a result of palladium disappearance from the catalyst, strongly adsorbed extraneous compounds and thermal rearrangement of the palladium on the surface of the catalyst. Rejuvenation procedures developed in the past involved a cleansing of the catalyst surface through extraction with liquids. The effectiveness of these procedures depends on the nature of the adsorbate; the palladium content and palladium dispersion are not restored. Furthermore, these rejuvenation procedures are cumbersome and impractical since they involve the removal of the catalyst from the process loop, an extraction process and return to the loop. Poor catalyst performance, as a result of acute or chronic catalyst poisoning, is usually remedied by higher than normal purging of catalyst and replacement with virgin catalyst. The latter is undesirable because it lowers the total amount of solids in the loop, which causes instability in the process operation. Furthermore, excessive catalyst purging is expensive due to high palladium conversion costs. A method which can restore the activity and the selectivity of the catalyst present in the process loop without the disadvantages mentioned above would be desirable and essential for capacity increase.
Other catalysts used in industrial processes also invariably experience deactivation. The deactivation can be caused by contaminants in the feedstock through formation of inhibitors on the surface of the catalyst, by thermal rearrangement of the active sites on the catalyst or by disappearance of metal from the support in the case of a supported catalyst.
In the phenol hydrogenation process, catalyzed by supported palladium, the catalyst also becomes deactivated in varying degrees. Thus, it has been observed that the catalyst present in the continuous process loop has only one-tenth the activity of the virgin make-up catalyst. The degree of deactivation can often times be correlated with impurities in the phenol feedstock. For instance, low oxidation state sulfur compounds when present in phenol are a prime cause for deactivation. Acetol, a known by-product in the cumene phenol process decomposes to carbon monoxide on the surface of the catalyst and thus deactivates the catalyst. Small amounts of iron and nickel entering the process with phenol and with hydrogen gas cause catalyst deactivation. Iron and nickel furthermore catalyze the undesirable conversion of phenol directly to cyclohexanol to a much greater extent than palladium does. Iron and nickel, therefore, lower the selectivity of the catalyst. Other agents such as organic and inorganic acids and amines are known to lower the activity and the selectivity of the catalyst.
The underlying reason for deactivation is simply the fact that one ingredient or another in the process is adsorbed on the active sites of the catalyst much more strongly than phenol. In the case of selectivity loss, the reason can be twofold: (1) the specific active sites adsorbing phenol are blocked more than different active sites adsorbing cyclohexanone or (2) other metals, such as iron or nickel, adsorbed on the catalyst convert phenol to cyclohexanol. When the expressions "ol" and "one" are used herein, they mean cyclohexanol and cyclohexanone, respectively.
The disappearance of palladium from the catalyst also leads to lower activity. The disappearance of palladium from the carbon support in the hydrogenation process is an established fact. While the catalyst supplied to the process contains 5% palladium, this level readily drops in the process to a lower level. The mechanism of disappearance is understood only in part. It is known that the small particle size fraction of the catalyst, containing a disproportionally high palladium content, is not retained in the continuous process loop by the centrifuges. Escape of the small particle size portion, therefore, results in a loss of palladium; but this does not adequately account for all of the palladium lost. It is speculated that two other mechanisms are involved. One of these is related to the observed presence of palladium salt, rather than palladium metal only, on the virgin catalyst. Presumably the palladium salt became dissolved into the process liquids and thus became lost from the catalyst. The other possible mechanism involves the dissolution of palladium as a palladium hydride encouraged by metal complexing agents such as amines present in the process liquids. Evidence for the latter is the fact that palladium has been found to plate out in the areas of the process having low or zero hydrogen pressure. A dissolved palladium hydride molecule forced to give up its hydrogen and not capable of reconstituting it will precipitate from the solution.
In summary, deactivation and loss of selectivity appear to be a result of loss of active sites through (1) a very tenacious masking process and (2) escape. On the basis of these conclusions, we have devised a procedure to restore the characteristics of favorable activity and selectivity in a used catalyst to any degree desirable.
By weak acid herein is meant any anion conjugated with an acid having a dissociation constant, pKa, greater than 3.00.